We bolted an old palm sander to the under side of our bridge last year in our Swamp theme, it was triggered when they hit the halfway point on the bridge......you could hear it all through the haunt hahahahaha. It did get great reactions though, it vibrated the whole bridge.

yea it's loud,
we used them on a wooded bridge that had "high voltage" lines around it...with electrical sounds. I'm not sure if that's what your after but it worked...
We even did the same gimmick on our hayride. Bolted the sanders under the school bus seats on our trailer in the "middle seats" b/c that's where everyone who is scared wants to sit.